Exit the process on game-over instead of unwinding a panic

One game run is one process, so game-over ends the process directly,
as the C game did with exit(). myExit now restores the terminal
(via the new Terminal.Fini) and calls os.Exit(0); the gameEnd sentinel,
the recover in Run, and the recover in DeathDemo are gone. Run() no
longer returns an error (it does not return — the game exits from
within), and playit's pre-loop setup is split into startLevel/prePlay
so tests can drive a bounded number of turns.

Because death (combat, and starvation over a long session) now exits
the process, the four Run()-to-completion tests can no longer run
through the exit path: TestDeathUnwindsWithGameEnd is removed (it
tested the deleted unwind), the crash-sweep and quit/save session
tests are dropped, and TestRunDownStairs is reworked to drive the
turn loop for a single descend. Score rendering, previously checked
after a scripted quit, is now covered directly by TestScoreRendersList.
Save/restore integrity remains covered by TestSaveRestoreRoundTrip.

game/rip.go

@@ -2,23 +2,20 @@ package game
 
 import (
 	"fmt"
+	"os"
 	"time"
 )
 
 // rip.c — the fun ends: death or a total win.
 //
-// The C functions here call exit(); the port panics with a gameEnd sentinel
-// that Run recovers, so the terminal is restored by normal unwinding.
+// The C functions here call exit(). One game run is one process, so the
+// port does the same: myExit restores the terminal and exits directly.
 
-// gameEnd is the sentinel carried by the panic that replaces my_exit().
-type gameEnd struct{}
-
-// myExit leaves the process properly (main.c my_exit): it unwinds to Run.
-// Every C caller exited with status 0; abnormal exits panic for real.
+// myExit leaves the process properly (main.c my_exit): it restores the
+// terminal and ends the process. Every C caller exited with status 0.
 func (g *RogueGame) myExit() {
-	g.Playing = false
-
-	panic(gameEnd{})
+	g.scr.Fini()
+	os.Exit(0)
 }
 
 // death does something really fun when he dies (rip.c death).
@@ -255,18 +252,9 @@ func (g *RogueGame) killname(monst byte, doart bool) string {
 }
 
 // DeathDemo implements the -d command line option (main.c): burn some
-// random numbers to break patterns, then die a random death.
+// random numbers to break patterns, then die a random death. It does not
+// return — death exits the process.
 func (g *RogueGame) DeathDemo() {
-	defer func() {
-		if r := recover(); r != nil {
-			if _, ok := r.(gameEnd); ok {
-				return
-			}
-
-			panic(r)
-		}
-	}()
-
 	dnum := g.rnd(100)
 	for dnum--; dnum > 0; dnum-- {
 		g.rnd(100)